The WLAN standards IEEE 802.11a/b/g are experiencing a successful development. This success is mainly based upon the use of these standards in enterprise networks, hot spots and at home, i.e. conference centers, railway stations, airports, hotels and so on. In these environments, Voice over Internet Protocol (VoIP) and Voice over WLAN (VoWLAN) are becoming attractive technologies with the main goal to reduce the communication costs by merging data and voice networks.
Offering real time handover is an essential requirement for VoWLAN and other real time services like video conferences and therefore the handover between two WLAN access points is an active subject of research because the handover time has to be drastically reduced in order to allow VoWLAN seamless mobility between two IEEE 802.11 access points.
The first handover phase consists in detecting as fast as possible whether a handover has to be carried out. The IEEE 802.11 standard e.g. only enables quite slow passive handover detection based on a so-called access point traffic or beacon listening, i.e. a mobile terminal detects a bad radio link quality with its current access point, which means that a handover has to be carried out, by means of power measurements of the user related traffic or beacon frames. In a WLAN network applying the IEEE 802.11 standard, data that are called beacon are more or less periodically broadcasted from an access point marking the beginning of a contention-free transaction period and enabling synchronization of all devices within the WLAN network with a time period called beacon interval, which has typically the length of 100 ms in case of current WLAN deployments. About data that are called user related traffic frame, no known interval can be considered, the interval depends on the traffic of the application.
Thus in case no data except this beacon is used for power measurements, the latency for the detection of the necessity of a handover will be in the range of 100 ms which does not offer an acceptable quality for real-time applications.
By means of power measurements of user related data that are exchanged between a mobile terminal and an access point, this detection procedure can be speeded up, but if no user related data are exchanged between a mobile terminal and an access point for a longer time, no power measurements can be performed and the detection whether a handover is necessary occurs quite late. As a consequence the handover procedure is quite long and real time services such as VoWLAN during mobility will be degraded.
The same problem as described above of course also occurs in other mobile networks than WLAN networks, e.g. in Global System for Mobile Communication (GSM) networks, as here user related data are exchanged e.g. over the traffic channel (TCH) between a mobile terminal and an access point, here called Base Transceiver Station (BTS), and these user related data are used for power measurements.